I am guessing you are using one of the ojp mods with weapon tags on playermodels? It that's the case could you tell the .sab file that the sword is two handed so it will be classified as a staff and placed on the back? I think the command in the .sab is: twohanded 1 : or something like that. There are plenty of saber files to check. Even if that works it may look bad on the back as well with how the tags may place the saber, such as too high or too low.